WO1998051916A1

WO1998051916A1 - Cooling systems suitable for use with gas turbines

Info

Publication number: WO1998051916A1
Application number: PCT/US1998/010036
Authority: WO
Inventors: William L. Kopko
Original assignee: Work Smart Energy Enterprises, Inc.
Priority date: 1997-05-16
Filing date: 1998-05-15
Publication date: 1998-11-19
Also published as: AU7492798A

Abstract

In a gas turbine system, passive cooling is used to reduce inlet air temperature below ambient wet-bulb temperature. One embodiment uses a regenerative indirect evaporative cooler (1) without moisture addition or mechanical refrigeration. A direct evaporative cooler (2) or mechanical cooling system (10, 11) may also be used downstream of the regenerative cooler. Further embodiments use liquid fuel (20) and ground or lake water (17) as cooling sources.

Description

COOLING SYSTEMS SUITABLE FOR USE WITH GAS TURBINES

Applicant claims the benefit of co-pending US provisional application Serial Number

60/046,675 filed on May 16, 1997.

BACKGROUND OF THE INVENTION

Field of the Invention

This invention relates to systems and methods of power generation, and in particular

relates to improvements in gas turbine power plant generation efficiency. Background and Prior Art

Gas turbines are a vital part of the world's power generating systems. Sales of new turbines amount to many billions of dollars per year and manufacturers have spent huge

amounts of money in optimizing turbine design for maximum efficiency and generating

capacity. Yet despite these huge expenditures, gas turbines have an inherent performance

limitation at high ambient air temperatures that significantly limits their generating capacity at

peaking conditions, which occur at times of maximum demand. Since most electric utilities

in the United States and many other areas of the world experience their peak generating demand in the summer, this limitation has a serious effect on the economics of gas-turbine

power plants.

Engineers have long recognized that the capacity and efficiency of gas turbines are

reduced with high inlet air temperatures. Typically turbine capacity decreases by .4%, and efficiency decreases by .1%, per degree Fahrenheit. These reductions in efficiency and capacity are especially important since they are greatest at high ambient temperatures that coincide with the maximum demand for power generating capacity.

The most common setup for cooling inlet air to turbines is a direct evaporative cooler. This system simply draws air across a wet pad, which lowers the air temperature by

evaporation. These systems are relatively simple and have a low initial cost. One problem

with this approach , however, is that the ambient wet-bulb temperature limits the minimum

air temperature leaving the cooler.

Another problem is that the enthalpy of the air is essentially constant in the

evaporation process. Constant enthalpy means that a direct evaporative cooler has virtually

no effect on the energy removed by a downstream cooling coil that cools the air below the

ambient dewpoint temperature. This feature greatly limits the use of direct evaporative

coolers with other cooling methods.

Ondryas, Wilson, Kawamato, and Haub, in their paper "Options in Turbine Power Augmentation Using Inlet Air Chilling," presented at the Gas Turbine and Aeroengine

Congress and Exposition, June 11-14, 1990 in Brussels, Belgium, provide an exhaustive list

of existing alternatives for gas-turbine inlet cooling. They discuss the use of chillers in

cooling inlet air for gas turbines. They show that electric or absorption chillers can be successfully used to cool inlet air, although such chillers require a large capital investment.

Ondryas et al. also discuss using indirect evaporative coolers to precool the air before it goes over the chilled water heat exchanger. The existing approach for these coolers is to

use a second flow of ambient air across a wet surface of an air-to-air heat exchanger. The dry

surface cools the primary air stream for the turbine. For conditions of 95 dry bulb, 71 ° F wet bulb air temperature, an existing evaporative cooler can cool the air to 74° F. This results

in a 40% reduction in cooling load on the chillers. U.S. Patent No. 5,203,161 describes a much more complicated system that uses a combination of chillers and desiccants for turbine inlet air cooling. The chief disadvantage of

this system is the high first cost.

U.S. Patent No. 4,418,527 describes an indirect-direct evaporative cooling system for use in cooling inlet air gas turbines. This system also uses waste heat from the turbine to

drive a distillation system that provides pure water to the evaporative cooler. An important limitation of this approach is that the indirect evaporative cooler cannot cool the air below the

ambient wet-bulb temperature in this configuration.

U.S. Patent No. Re. 20,449 (originally Pat. No. 2,057,938) describes a regenerative evaporative cooling system. This system recirculates a portion of the air exiting the dry side

of an indirect evaporative cooler for use on the wet side of the heat exchanger. The air exiting the dry side of the heat exchanger can approach the inlet dewpoint temperature in this

system. Other patents, such as U.S. Pat. No. 4,854,129, also use this principle, but they are

intended for space cooling. None of these prior art systems have been used for inlet air cooling for turbines.

SUMMARY OF THE INVENTION

Accordingly, there exists a need in the art for improvement in efficient and low cost

cooling of turbine inlet air streams. A major objective of the present invention is to provide a

method for cooling inlet air for gas turbines that minimizes first cost while maximizing

performance advantages. A related objective of the invention is to maximize the use of

passive cooling techniques that minimize the need for mechanical refrigeration equipment. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will become more clearly understood from the following detailed description in conjunction with the accompanying drawings, wherein:

Fig. 1 is a block diagram of a gas-turbine power plant system according to a first

embodiment of the invention, using a regenerative indirect evaporative cooler for inlet air;

Fig. 2 is a block diagram of a gas-turbine power plant system according to a second

embodiment of the invention, combining a mechanical cooling system with Fig. 1;

Fig. 3 is a block diagram of a gas-turbine power plant system according to a third embodiment of the invention, using a source of cold water as a cooling medium; and Fig. 4 is a block diagram of a gas-turbine power plant system according to a fourth

embodiment of the invention, using liquid fuel evaporation to cool inlet air to a gas-turbine power plant.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 1 shows one preferred embodiment of the present invention. A heat exchanger 1 acts as a regenerative indirect evaporative cooler to cool a flow of incoming air 3 to a

temperature near the dewpoint without adding moisture to the air stream. The air leaving the heat exchanger 1 splits into two flowstreams, 4 and 5. The first portion 4 of the air stream

that leaves the cooler continues through a direct evaporative cooler 2, which further cools and

humidifies the air to near its wet bulb temperature. The second portion 5 of the air stream flows back into the wet side of cooler 1 in an approximately counterflow direction to the

incoming air stream 3. Fan 9 draws exhaust air steam 6, out of the regenerative indirect evaporative cooler 1. The direct evaporative cooler 2 may include drift eliminators if air velocities are sufficiently high. A filter (not shown) located in the incoming air stream,

upstream of the indirect evaporative cooler 1 also may be necessary in areas with high amounts of dust or lint in the air. Cooled air stream 8 from the direct evaporative cooler 2 enters gas-turbine power plant 7. The gas-turbine power plant typically comprises a compressor that supplies high-pressure air to a burner which heats the air before it enters a turbine. Power is extracted from the hot expanding gases by exhausting the gases through the

turbine which causes the turbine to rotate, and the rotating turbine typically drives an electric

current generator. The gas-turbine power plant 7 may be part of a combined-cycle power plant which would include a steam system that is driven by thermal energy from the turbine

exhaust gases. The use of the direct evaporative cooler 2 is optional and can be eliminated in

cases where the temperature of air leaving the indirect evaporative cooler 1 is within a few

degrees of the dewpoint.

While counterflow is the optimum heat exchanger configuration from a thermodynamic viewpoint, practical considerations related to design of flow paths may require the use of multipass crossflow or even pure crossflow. The required flow rate of the secondary air is approximately 25 to 35 percent of the primary air flow through the heat

exchanger. In order for the primary air to closely approach the dewpoint temperature, the

temperature change of the secondary air has to be less than that of the primary air. The

quantity of secondary air is related to the change in enthalpy of air per degree of wet bulb

temperature change compared to the specific heat of air.

Design of the heat exchanger for the evaporative cooler favors the use of low-cost plastic material. Corrugated plastic panels are inexpensive and corrosion resistant. An

example of possible material is a polypropylene panel sold under the Coroplast brand name. The internal channels of these panels provide a large surface area for the dry side of the heat

exchanger, and the external face is exposed to the secondary air. The panels can be stacked

and bonded together either mechanically or by welding. One option for providing necessary moisture to the secondary air is to inject a mist of water into the secondary air stream. Another option is to cover the exterior with a thin fibrous material to provide a wetable

surface. Trickling water over this face then provides a wet surface on the secondary air side of the heat exchanger. Numerous other heat exchanger configurations are possible and available from the prior art.

For the conditions of 95° F dry bulb and 71 ° F wet bulb, the indirect evaporative

cooler system of Fig. 1 can cool the air to within a few degrees of the dewpoint temperature

of 59° F. In contrast, a conventional indirect evaporative cooler would be limited to about 74° F. The result is roughly a 50% increase in the temperature change that is possible without the need for mechanical cooling or the addition of moisture. Since no moisture is added, the cooling results in a lowering of the enthalpy of the air stream, which also reduces

the needed cooling from a downstream heat exchanger.

Fig. 2 shows a second preferred embodiment of the invention, which uses chilled water or chilled brine in a cooling coil to further reduce the inlet air temperature. The same regenerative indirect evaporative cooler components as shown in Fig. 1 are indicated by like reference numerals. According to this embodiment, a water-to-air heat exchanger 10 is

provided between the indirect evaporative cooler 1 and the turbine 7. This water-to-air heat

exchanger may be a direct-contact type or it may be of a closed-loop (indirect contact)

configuration. The closed-loop configuration has the advantage of eliminating fouling

problems associated with dirt from the air. A chilled water line 12 supplies cold water to the

heat exchanger 10, which exits through water line 13. Chilled water pump 14 pumps the

water back to chiller 11, which cools the water. The chiller 11 is preferably an absorption chiller that is driven by waste heat from turbine exhaust gas 15. Alternatively, a vapor

compression chiller that uses an electrical input can be used, which would not use the exhaust air from the turbine. While water is the preferred heat transfer fluid in warm climates, use of a brine may be necessary in areas that experience freezing temperatures. The advantage of

this arrangement is that it can achieve much colder temperatures than those available with an evaporative cooler alone.

Fig. 3 illustrates a third preferred embodiment according to the invention, that uses cold ground water or lake water for cooling. Cold water source 17 supplies water through

line 18 to a water-to-air heat exchanger 10 which cools ambient air stream 3. The cooled air stream 8 exits the heat exchanger 10 and enters gas-turbine power plant 7. After exiting the heat exchanger, cooling water is disposed to sink 16. The water-to-air heat exchanger 10

would preferably have direct contact between the water and air to minimize cost and to reduce potential freezing problems and reduce cost. The entering water temperature should be lower than the ambient air temperature and preferably lower than the ambient dewpoint

temperature.

This system requires a large source of cold water 17 and a sink 16 for disposing of the

water. A good location for this embodiment would be over an aquifer that is recharged by a

river. Ground water can be pumped from the aquifer, used for cooling purposes, and

discharged into the river.

Another option is to use the water from the bottom of a deep lake, ocean, or other

large body of water. In climates where freezing conditions occur for a significant portion of

the year, bottom temperatures around 40° F are typical in deep reservoirs. This cold bottom

water from reservoirs is frequently used to drive hydroelectric turbines and is thus readily

available for cooling use. Adding heat to this water may provide environmental benefits

since the icy temperatures downstream of a dam can inhibit the growth of organisms that

would normally be present in a free-flowing river. In coastal regions deep ocean water may also be suitable for this purpose. The use of evaporative precooling is not necessary in these systems, but may be desirable to reduce the need for cooling water if only limited water is available. Systems using naturally occurring sources of cold water are most suitable for new

installations that have a large degree of flexibility in site selection. Fig. 4 shows a fourth preferred embodiment of the invention, wherein a liquid fuel is

evaporated to provide additional cooling. Like elements of Fig. 1 are given like reference

numerals in Fig. 4. The preferred fuel for this embodiment is liquid ammonia. The fuel

flows from a storage tank 20 through line 21 and control valve 26 to evaporator coil 22. The control valve 26 regulates the flow of fuel to the evaporator coil. The liquid fuel evaporates in

this coil to cool the inlet air stream 4. The cooled air stream 8 enters the turbine 7. The vaporized fuel then passes out of coil 22 through line 23 to compressor 24. Line 25 connects

the compressor to a burner (not shown) inside turbine 7. The purpose of the compressor 24 is to raise the pressure of the fuel vapor to a level above that of the air at the burner. The

compressor may be eliminated if the vapor pressure of the fuel in the evaporator coil is above that necessary to supply the burner in the gas-turbine power plant.

The equation below gives the amount of cooling available from the evaporation of

fuel:

Δh_aιr = (HR / m_aιr) x (h_fg / h_f) (1)

where

Δh_aιr = cooling in Btu lbm of air,

HR = heat rate of the turbine in Btu/hr/kw,

m_aιr = mass flow rate of air per unit of output power in lbm hr/kw,

h_fg = heat of vaporization of the fuel in Btu/lbm, and h_f = heat of combustion of the fuel in Btu/lbm.

For ammonia and a heat rate of 10000 Btu/lbm and flow rate of air of 36 lbm/hr/kw,

the available cooling according to equation (1) is 14 Btu lbm of air. This cooling could

reduce inlet air temperatures by as much as 60° F for dry air, although 20° F is a more

reasonable number for moist air.

Since ammonia burns very cleanly with nitrogen and water as combustion products,

this system also has the advantage of lowering pollution emissions. Peak generating periods

correspond closely to peak smog periods, so the lower emissions may be especially valuable.

The high cost of ammonia compared to that of natural gas would limit this system to short

periods of operation. Other possible fuels include propane, ethane, and dimethyl ether, but

they have the disadvantage of a smaller heat of vaporization compared to their heat of

combustion, which reduces the amount of possible cooling. An added benefit of this system

is the availability of a backup fuel, which improves reliability and can give an opportunity for

lower interruptible gas rates.

The invention having been described, it will apparent to those skilled in the art that

the same may be varied in many ways without departing from the spirit and scope of the

invention. All such modifications are intended to be covered within the scope of the

following claims.

Claims

What is claimed is:

1. A method for improving the performance of a gas-turbine power plant comprising

the steps of:

providing an ambient air flow stream;

passively cooling said air flow stream to a temperature below the ambient wet

bulb temperature without the addition of moisture to said air stream and without the use of

mechanical refrigeration; and

supplying at least a portion of said cooled air stream to the inlet of said gas-

turbine power plant.

2. The method of claim 1 wherein the step of cooling said ambient air stream

comprises the steps of:

removing heat from said ambient air stream by placing said ambient air stream in

contact with a dry heat exchange surface;

splitting said ambient air stream after contact with said dry heat exchange surface into

a first portion that enters a turbine of said power plant and a second portion which moves into

contact with a wet heat exchange surface; and

exchanging heat between said dry heat exchange surface and said wet heat exchange

surface.

3. The method of claim 2 further comprising the steps of:

evaporating a liquid fuel in a heat exchanger that is in contact with said first portion of

said air stream prior to entry into said turbine; and

burning the resulting fuel vapor in said gas-turbine power plant.

4. The method of claim 3, wherein said fuel is ammonia.

5. The method of claim 2, wherein the step of cooling said ambient air stream comprises the

further step of:

supplying water that has a temperature below the ambient wet-bulb temperature to a

heat exchanger in contact with said air stream.

6. The method of claim 5 wherein said water is drawn from the ground.

7. The method of claim 5 wherein said water is drawn from deep portions of a large body of

water.

8. A gas-turbine power plant system, comprising:

a gas-turbine power plant; and

cooling means for providing a cooled stream of ambient air to said gas-turbine power

plant, including an indirect evaporative cooler having a dry heat exchange surface and a wet

heat exchange surface, wherein an ambient air stream is passed over said dry heat exchange

surface to cool said ambient air stream and thereafter is split into first and second portions,

said first portion being supplied to said gas-turbine power plant and said second portion being

passed over said wet heat exchange surface.

9. The gas-turbine power plant system of claim 8, further comprising:

means for storing liquid fuel;

means in contact with said first portion of said air stream prior to entry into said

turbine, for evaporating said liquid fuel so as to further cool said first portion of said air

stream; and

means for burning the resulting fuel vapor in said gas-turbine power plant.

10. The gas-turbine power plant system of claim 8, further comprising: a heat exchanger for further cooling said first portion of said air stream.

11. The gas-turbine power plant system of claim 10, wherein said heat exchanger is a direct

evaporative cooler.

12. The gas-turbine power plant system of claim 10, wherein said heat exchanger is water-

to-air heat exchanger.

13. The gas-turbine power plant system of claim 12, wherein said water-to-air heat

exchanger is a direct contact heat exchanger.

14. The gas-turbine power plant system of claim 12, wherein said water-to-air heat

exchanger is an indirect closed-loop heat exchanger.

15. The gas-turbine power plant system of claim 12, wherein said water-to-air heat

exchanger receives a supply of cooling water from a cold water source, and discharges

cooling water to a sink.

16. A method for improving the performance of a gas-turbine power plant comprising the

steps of:

providing an ambient air flow stream;

cooling said air flow stream to a temperature below the ambient wet bulb temperature

by placing said ambient air stream in contact with a dry heat exchange surface;

contact with a wet heat exchange surface; and

surface.

17. The method of claim 16 further comprising the step of: evaporating water into the first portion of the air stream so as to further lower its

temperature before it enters said turbine.

18. The method of claim 16 further comprising the step of:

mechanically cooling said first portion of the air stream before it enters said turbine.

19. The method of claim 18 where said step of mechanically cooling comprises the steps of

cooling a liquid with a chiller and supplying said cooled liquid to a heat exchanger that

is in contact with said first portion of the air stream.

20. The method of claim 19 wherein said chiller is an absorption chiller that is driven by

thermal energy from the exhaust air stream from said gas-turbine power plant.

21. The method of claim 19 wherein said liquid is water, the temperature of said water is

below the ambient dewpoint temperature, and said heat exchanger allows direct contact

between the water and said air stream.